Downy mildew is one of the most serious grapevine (Vitis vinifera) diseases in the world. It is caused by the biotrophic oomycete Plasmopara viticola, which can attack all green parts of the grapevine (Gessler et al., 2011). These days, control of downy mildew relies mainly on frequent applications of chemical fungicides in conventional agriculture or copper in organic production (Wong et al., 2001; Gessler et al., 2011). Similarly, potato late blight and tomato late blight are serious plant diseases caused by the oomycete Phytophthora infestans, which is also routinely treated with e.g. fungicides as mentioned above (Fry and Goodwin, 1997; Mizubuti et al., 2007).
Growing concerns about the negative impact of chemical fungicides (chemical agents) and copper on agricultural soils are driving the search for new, low-impact, active components to e.g. control Plasmopara viticola and Phytophthora infestans. Certain microorganisms with favourable toxicological and eco-toxicological profiles offer potential solutions.
Although in recent years several bacterial strains have been selected for biological control of plant diseases caused by fungi and oomycetes (Lugtenberg and Kamilova, 2009), very few have been identified for the control of Plasmopara viticola. A strain of Erwinia herbicola has been shown to inhibit germination of Plasmopara viticola sporangia in vitro (Tilcher et al., 1994) and some bacterial strains belonging to Erwinia, Pseudomonas and other genera have yielded promising results (Tilcher et al., 2002), but no follow-up studies on these bacteria have been reported in spite of the enormous efforts being made to find alternatives to chemical-based fungicides to control Plasmopara viticola. On the contrary, several scientific works were dealing with the evaluation of bacterial strains for the control of Phytophthora infestans (De Souza et al., 2003; Lourenco et al., 2006; Zakharchencko et al., 2011) but, at the moment, few are the bacteria-based plant protection products that can be exploited for the biocontrol of this plant pathogenic oomycete.
Certain bacteria of the genus Lysobacter have been brought in connection with e.g. soil suppressiveness, high production of lytic enzymes, or antibiotic production. Also, the genus Lysobacter (Christensen and Cook, 1978) includes bacterial species with a potential for biological control of plant pathogens (Hayward et al., 2010). Moreover, some Lysobacter strains have already been shown to actively protect plants from attack by soil-borne oomycetes. For example, Lysobacter enzymogenes strain 3.1T8, isolated from rockwool, inhibits mycelial growth of Phytophthora capsici, Pythium ultimum and Pythium aphanidermatum in vitro. Production of extracellular proteases, lipases and unidentified biosurfactants and antifungal molecules by Lysobacter enzymogenes strain 3.1T8 is involved in the control of infections caused by Pythium aphanidermatum on cucumber plantlets (Folman et al., 2003, 2004). Lysobacter sp. strain SB-K88 synthesises Xanthobaccin A, B and C, macrocyclic lactams which are highly effective in vitro against Aphanomyces cochlioides, Phytophthora vignae f. sp. adzukicola and Pythium ultimum (Nakayama et al., 1999). Application of strain SB-K88 and the compound Xanthobaccin A to the seeds of sugar beet suppressed damping-off in natural soil hosting populations of Pythium spp. (Homma et al., 1993; Nakayama et al., 1999). Furthermore, Lysobacter sp. SB-K88 directly antagonises the mycelium of A. chloclioides by directly attacking the cell wall of this oomycete (Islam et al., 2005). Strain YC5194, type strain of the species Lysobacter capsici, is closely related genetically to strain SB-K88 (Park et al., 2008; Puopolo et al., 2010). It was isolated from the rhizosphere of the pepper plant (Capsicum annuum) and inhibits the growth of Pythium ultimum and other phytopathogenic fungi in vitro (Park et al., 2008). Another member of this species, Lysobacter capsici strain PG4, reduces mycelial growth of several fungi and oomycetes in vitro and when applied to tomato seeds controls tomato foot and root rot caused by Fusarium oxysporum f. sp. radicis-lycopersici (Puopolo et al., 2010).
However, it has been difficult to find (new) bacterial antagonists to be used as plant protection products. The main reasons for failure in the selection of new bacterial antagonists are the poor survival rate of bacterial biocontrol agents on leaves and their incompatibility with copper-based fungicides, which does not, for example, allow microorganisms to be integrated into a low-dose copper-based strategy (Dagostin et al., 2011).
Accordingly, and despite extensive research and progress made so far, there remains a need for bacterial plant protection products with advantageous features such as persistence on plant leaves. Especially, there remains a need in the art for alternative or improved, respectively, treatments of fungal and oomycete diseases, such as diseases caused by Plasmopara viticola or Phythophthora infestans. This is particularly so in view of the various well-known advantages of plant protection products that include microorganisms when compared to standard (chemical) plant protection products, particularly if they are considered nonpathogenic and non-infectious to humans and animals. Further, there remains a need for identifying bacterial plant protection products that are compatible with standard (chemical) plant protection products such as copper-based plant protection products such that the use of such non-biological plant protection products can be decreased e.g. by employing combination treatments.
In view of the above, the present inventors identified a new Lysobacter strain, Lysobacter capsici AZ78, and explored the ability of Lysobacter capsici AZ78 to persist in the grapevine and tomato phyllosphere and at the same time to control e.g. Plasmopara viticola and Phytophthora infestans in vivo. In addition, the present inventors show that AZ78 is resistant to copper ions and can be applied together with copper-based fungicides/plant protection products in low doses to control grapevine downy mildew and tomato late blight. The present inventors also assessed its tolerance to abiotic stresses associated with starvation, high temperatures and UV irradiation. Based on these characteristics, Lysobacter capsici AZ78 may be exploited by means of new, integrated, disease-management strategies.
Accordingly, the present inventors have succeeded in finding new advantageous and effective bacteria of Lysobacter capsici species, which can be used to control various plant diseases. Moreover, with the present studies, the present inventors have surprisingly found that a new strain of Lysobacter capsici shows resistance to copper and, moreover, surprisingly allows an advantageous combination treatment of various pathogenic fungi and oomycetes.